An extreme draft is not desirable in the spinning of filling yarns either on a frame or mule, although yarn from the latter machines is usually spun from coarser roving than would be possible on a filling frame, and it would seem preferable to spin same from roving with a medium draft, or, from double roving, and particularly so for fine sheetings, cambrics and other styles of cloth, that require finer filling than the warp and which have a higher count or pick, than sley.
Source: wiktionary